Time

Discussions of the nature of time, and of various issues related to
time, have always featured prominently in philosophy, but they have
been especially important since the beginning of the 20th Century.
This article contains a brief overview of some of the main topics in
the philosophy of time — Fatalism; Reductionism and Platonism
with respect to time; the topology of time; McTaggart's arguments;
The A Theory and The B Theory; Presentism, Eternalism, and The
Growing Universe Theory; time travel; and the 3D/4D controversy
— together with some suggestions for further reading on each
topic, and a bibliography.

Note: This entry does not discuss the consciousness, perception,
experience, or phenomenology of time. An historical overview and
general presentation of the various views is available in the entry
on temporal consciousness.
Further coverage can be found in the SEP entry on
the experience and perception of time.
For those interested specifically in phenomenological views,
see the entries on
Husserl (Section 6), and
Heidegger (Section 2: Being and Time).

A good deal of work in the philosophy of time has been produced by
people worried about Fatalism, which can be understood as the thesis
that whatever will happen in the future is already unavoidable (where
to say that an event is unavoidable is to say that no human
is able to prevent it from occurring). Here is a typical argument for
Fatalism.

(1)

There exist now propositions about everything that might happen in
the future.[1]

(2)

Every proposition is either true or else false.

(3)

If (1) and (2), then there exists now a set of true propositions
that, taken together, correctly predict everything that will happen in
the future.

(4)

If there exists now a set of true propositions that, taken
together, correctly predict everything that will happen in the future,
then whatever will happen in the future is already unavoidable.

(5)

Whatever will happen in the future is already unavoidable.

The main objections to arguments like this have been to premises (2)
and (4). The rationale for premise (2) is that it appears to be a
fundamental principle of semantics, sometimes referred to as The
Principle of Bivalence. The rationale for premise (4) is the claim
that no one is able to make a true prediction turn out false.

A proper discussion of Fatalism would include a lengthy consideration
of premise (4), and that would take us beyond the scope of this
article. For our purposes it is important to note that many writers
have been motivated by this kind of argument to deny Bivalence.
According to this line, there are many propositions — namely,
propositions about matters that are both future and contingent
— that are neither true nor false right now. Take, for example,
the proposition that you will have lunch tomorrow. On this view, that
proposition either has no truth value right now, or else has the
value indeterminate. When the relevant time comes, and you
either have lunch or don't, then, on the view in question, the
proposition that you have lunch on the relevant day will come to be
either true or false (as the case may be), and from then on that
proposition will forever retain its truth
value.[2]

The view that Bivalence is false, and that, in particular, there are
sometimes propositions about the future that are neither true nor
false, is sometimes referred to as the “Open Future”
response to arguments for Fatalism. One important presupposition of
the Open Future response is that it makes sense to talk about a
proposition's having a truth value at a time, and that, moreover, it
is possible for a proposition to have different truth values at
different times. Thus, the Open Future response to arguments for
Fatalism entails the following semantical thesis.

The Tensed View of Semantics:

Propositions have truth values at times rather than just
having truth values simpliciter.

The fundamental semantical locution is ‘p is
v at t’ (where the expression in place of
‘p’ refers to a proposition, the expression in
place of ‘v’ refers to a truth value, and the
expression in place of ‘t’ refers to a time).

It is possible for a proposition to have different truth values at
different times.

The Tensed View of Semantics can be contrasted with the following
semantical view.

The Tenseless View of Semantics:

Propositions have truth values simpliciter rather than
having truth values at times.

The fundamental semantical locution is ‘p is
v’ (where the expression in place of
‘p’ refers to a proposition and the expression in
place of ‘v’ refers to a truth value).

It is not possible for a proposition to have different truth values
at different times.

Other views that have (at least sometimes) been associated with the
Open Future response to Fatalism include Taking Tense Seriously and
The Growing Universe Theory, which will be discussed below.

What if one day things everywhere ground to a halt? What if birds
froze in mid-flight, people froze in mid-sentence, and planets and
subatomic particles alike froze in mid-orbit? What if all change,
throughout the entire universe, completely ceased for a period of,
say, one year? Is such a thing possible?

If the answer to this last question is Yes — if it is possible
for there to be a period of time during which nothing changes,
anywhere (except, perhaps, for the pure passage of time itself, if
there is such a thing) — then it is possible that a worldwide
“freeze” will occur between the time you finish reading
this sentence and the time you start the next sentence. In fact, if
it's possible for there to be a period of time without change, then
it may well be that a million years have passed since you finished
reading the last sentence.

The question of whether there could be time without change has
traditionally been thought to be closely tied to the question of
whether time exists independently of the events that occur in time.
For, the thinking goes, if there could be a period of time without
change, then it follows that time could exist without any events to
fill it; but if, on the other hand, there could not be a period of
time without change, then it must be that time exists only if there
are some events to fill it.

Aristotle and others (including, especially, Leibniz) have argued
that time does not exist independently of the events that occur in
time. This view is typically called either “Reductionism with
Respect to Time” or “Relationism with Respect to
Time,” since according to this view, all talk that appears to
be about time can somehow be reduced to talk about temporal relations
among things and events. The opposing view, normally referred to
either as “Platonism with Respect to Time” or as
“Substantivalism with Respect to Time” or as
“Absolutism with Respect to Time,” has been defended by
Plato, Newton, and others. On this view, time is like an empty
container into which things and events may be placed; but it is a
container that exists independently of what (if anything) is placed
in it.

Why would someone endorse the reductionist view about time?
Historically, two main arguments have played the biggest roles in
convincing people. One is conceptual: time, according to this
argument, is by definition nothing more than a system of temporal
relations among things and events, so that the idea of a period of
time without change turns out to be incoherent. The other main
argument for Reductionism is epistemological: we could never have any
reason, according to this argument, to posit a period of empty time;
and, moreover, even if there were such a period, we would not have
any way of knowing about either its existence or its length.

What about Platonism with Respect to Time — why would someone
endorse that view? One reason is that the empty container metaphor
has a lot of intuitive appeal. (This is no doubt true of both the
temporal and spatial versions of Platonism.) And another reason is
that some people do not find the main arguments against Platonism
with Respect to Time compelling. For example, it has been suggested
by Sydney Shoemaker that there are possible circumstances in which it
would make perfect sense to posit periods of empty time, and even to
claim to know just how long those periods are.

Here is a simplified version of Shoemaker's argument. Consider a
small, spatially finite possible world that is divided into three
zones, A, B, and C. In Zone A, there is a complete freeze — a
cessation of all change — for one hour every 2 years. These
local freezes in Zone A are preceded by a short period in which every
object in A takes on a reddish glow (observable to the occupants of
all three zones), while at the same time a temporary force field
develops at the boundary of Zone A, preventing anything from entering
or exiting that zone during the freeze. While the freeze in Zone A is
taking place, Zone A appears to those in Zones B and C to be pitch
black, since no light can enter or exit the frozen zone; but as soon
as the local freeze in Zone A is over, the people in the other two
zones can again see everything in Zone A, and can in fact see those
things resuming their normal behaviors without missing a beat. To
those who remain in Zone A for the freeze, it appears that the
reddish glowing and the development of the force field are
immediately followed, not by any cessation of change, but, instead,
by a large number of sudden and discontinuous changes in the other
two zones.

Meanwhile, In Zone B there is a similar freeze for one hour every 3
years, and in Zone C there is a freeze for one hour every 5 years.
The inhabitants of this strange world quickly become aware of the
local freezes, and they have no trouble calculating the “freeze
function” for each of the three zones. What's more, they also
calculate that there is a global freeze — a period during which
each one of the three zones undergoes a local freeze — exactly
once every 30 years. Whenever a global freeze occurs, of course, no
one is able to see any frozen objects or blacked-out zones, since
everyone and everything is frozen at the same time. But the reddish
glowing and the development of temporary force fields that precede
each world-wide freeze are observable to everyone; and so the global
freeze times come to be celebrated by “empty time
parties” all over the world.

No doubt the inhabitants of this unusual world could come up with a
theory that explains the local freezes in a way that doesn't posit
any empty time. For they could theorize that in Zone A there is a
local freeze every two years, except for the 30th year, when there is
no freeze; and similarly for the other zones. But such a theory would
involve freezing functions that are more complicated than those that
entail a global freeze every 30 years.

What is this thought experiment supposed to show? Well, it can't be
taken to show that global freezes are possible, because (at least the
way the story has been told here) they are simply a stipulated detail
of the story, and we can't show that something is possible merely by
stipulating that it is the case in some possible world. What the
thought experiment does seem to show, however, is that it is possible
for rational beings to have at least some evidence for the existence
of periods of empty time in their world. For we can describe the
possible world of the thought experiment in a neutral way that
specifies how things in the world appear to its denizens, without
specifying whether the real freeze functions for Zones A, B, and C
are the simpler ones described above that entail a global freeze
every 30 years or the more complicated ones that do not have that
entailment. And a possible world that appears this way to its
inhabitants is surely a world in which those inhabitants have some
reason to take seriously the possibility that there are periods of
empty time in their world, that they know when those periods occur,
and even that they know exactly how long the periods of empty time
last.

Reductionism with Respect to Time and Platonism with Respect to Time
have spatial analogues, and the views about time have traditionally
been taken to stand or fall with their spatial counterparts. Indeed,
although there is considerable controversy over the degree to which
time is similar to the dimensions of space, the Reductionism vs.
Platonism dispute is widely thought to be one area in which the two
dimensions are perfectly analogous. (But it is worth noting that if
Shoemaker's argument is sound, then this conventional wisdom should
perhaps be challenged. For it does not appear that there will be
anything like a spatial analogue of that argument.)

It's natural to think that time can be represented by a line. But a
line has a shape. What shape should we give to the line that
represents time? This is a question about the topology, or structure,
of time.

One natural way to answer our question is to say that time should be
represented by a single, straight, non-branching, continuous line
that extends without end in each of its two directions. This is the
“standard topology” for time. But for each of the
features attributed to time in the standard topology, two interesting
questions arise: (a) does time in fact have that feature? and (b) if
time does have the feature in question, is this a necessary or a
contingent fact about time?

Questions about the topology of time appear to be closely connected
to the issue of Platonism versus Reductionism with Respect to Time.
For if Reductionism is true, then it seems likely that time's
topological features will depend on contingent facts about the
relations among things and events in the world, whereas if Platonism
is true, so that time exists independently of whatever is in time,
then time will presumably have its topological properties as a matter
of necessity. But even if we assume that Platonism is true, it's not
clear just what topological properties should be attributed to time.

Consider the question of whether time should be represented by a line
without a beginning. Aristotle has argued (roughly) that time cannot
have a beginning on the grounds that in order for time to have a
beginning, there must be a first moment of time, but that in order to
count as a moment of time, that allegedly first moment would have to
come between an earlier period of time and a later period of time,
which is inconsistent with its being the first moment of time.
(Aristotle argues in the same way that time cannot have an end.)

It is also worth asking whether time must be represented by a single
line. Perhaps we should take seriously the possibility of time's
consisting of multiple time streams, each one of which is isolated
from each other, so that every moment of time stands in temporal
relations to other moments in its own time stream, but does not bear
any temporal relations to any moment from another time stream.
Likewise we can ask whether time could correspond to a branching
line, or to a closed loop, or to a discontinuous line. And we can
also wonder whether one of the two directions of time is in some way
priveleged, in a way that makes time itself asymmetrical.

Suggestions for Further Reading: On the beginning and
end of time: Aristotle, Physics, Bk. VIII; Kant, The
Critique of Pure Reason, esp. pp. 75ff; Newton-Smith 1980, Ch. V;
Swinburne 1966. On the linearity of time: Newton-Smith 1980, Ch. III;
Swinburne 1966, 1968. On the direction of time: Price 1994, 1996;
Savitt 1995; and Sklar 1974. And finally, on all of these topics:
Newton-Smith 1980.

In a famous paper published in 1908, J.M.E. McTaggart argued that
there is in fact no such thing as time, and that the appearance of a
temporal order to the world is a mere appearance. Other philosophers
before and since (including, especially, F.H. Bradley) have argued
for the same conclusion. We will focus here only on McTaggart's
argument against the reality of time, which has been by far the most
influential.

McTaggart begins his argument by distinguishing two ways in which
positions in time can be ordered. First, he says, positions in time
can be ordered according to their possession of properties like
being two days future, being one day future,
being present, being one day past, etc. (These
properties are often referred to now as “A properties.”)
McTaggart calls the series of times ordered by these properties
“the A series.” But he says that positions in time can
also be ordered by two-place relations like two days earlier
than, one day earlier than, simultaneous with,
etc. (These relations are now often called “B
relations.”) McTaggart calls the series of times ordered by
these relations “the B series.”

(An odd but seldom noticed consequence of McTaggart's
characterization of the A series and the B series is that, on that
characterization, the A series is identical to the B series. For the
items that make up the B series (namely, moments of time) are the
same items that make up the A series, and the order of the items in
the B series is the same as the order of the items in the A series;
but there is nothing more to a series than some specific items in a
particular order.)

In any case, McTaggart argues that the B series alone does not
constitute a proper time series. I.e., McTaggart says that the A
series is essential to time. His reason for this is that change (he
says) is essential to time, and the B series without the A series
does not involve genuine change (since B series positions are forever
“fixed,” whereas A series positions are constantly
changing).

McTaggart also argues that the A series is inherently contradictory.
For (he says) the different A properties are incompatible with one
another. (No time can be both future and past, for example.)
Nevertheless, he insists, each time in the A series must possess all
of the different A properties. (Since a time that is future will be
present and past, and so on.)

One response to this argument that McTaggart anticipates involves
claiming that it's not true of any time, t, that t
is both future and past. Rather, the objection goes, we must say that
t was future at some moment of past time and will be past at
some moment of future time. But this objection fails, according to
McTaggart, because the additional times that are invoked in order to
explain t's possession of the incompatible A properties must
themselves possess all of the same A properties (as must any further
times invoked on account of these additional times, and so on ad
infinitum). Thus, according to McTaggart, we never resolve the
original contradiction inherent in the A series, but, instead, merely
generate an infinite regress of more and more contradictions.

Since, according to McTaggart, the supposition that there is an A
series leads to contradiction, and since (he says) there can be no
time without an A series, McTaggart concludes that time itself,
including both the A series and the B series, is unreal.

Philosophers like McTaggart who claim that time is unreal are aware
of the seemingly paradoxical nature of their claim. They generally
take the line that all appearances suggesting that there is a
temporal order to things are somehow illusory.

Needless to say, despite arguments such as McTaggart's, many
philosophers have remained convinced of the reality of time (for it
certainly seems like there is a temporal order to the world). But a
number of philosophers have been convinced by at least one part of
McTaggart's argument, namely, the part about the contradiction
inherent in the A series. That is, some philosophers have been
persuaded by McTaggart that the A series is not real, even though
they have not gone so far as to deny the reality of time itself.
These philosophers accept the view (sometimes called “The B
Theory”) that the B series is all there is to time. According
to The B Theory, there are no genuine, unanalyzable A properties, and
all talk that appears to be about A properties is really reducible to
talk about B relations. For example, when we say that the year 1900
has the property of being past, all we really mean is that 1900 is
earlier than the time at which we are speaking. On this view, there
is no sense in which it is true to say that time really passes, and
any appearance to the contrary is merely a result of the way we
humans happen to perceive the world.

The opponents of The B Theory accept the view (often referred to as
“The A Theory”) that there are genuine properties such as
being two days past, being present, etc.; that
facts about these A properties are not in any way reducible to facts
about B relations; and that times and events are constantly changing
with respect to their A properties (first becoming less and less
future, then becoming present, and subsequently becoming more and
more past). According to The A Theory, the passage of time is a very
real and inexorable feature of the world, and not merely some
mind-dependent phenomenon.

(It is worth noting that some discussions of these issues employ
terminology that is different from the A series/B series terminology
used here. For example, some discussions frame the issue in terms of
a question about the reality of tense (roughly, the
irreducible possession by times, events, and things of genuine A
properties), with A Theorists characterized as those who affirm the
reality of tense and B Theorists characterized as those who deny the
reality of tense.)

The A Theorist is normally happy to concede McTaggart's claim that
there can be no time without an A series, but the typical A Theorist
will want to reject the part of McTaggart's argument that says that
the A series is inherently contradictory. For the typical A Theorist
will deny McTaggart's claim that each time in the A series must
possess all of the different A properties. That is, she will deny
that it is true of any time, t, that tis
past, present, and future. Instead, she will insist, the closest
thing to this that can be true of a time, t, is (for
example) that twas future, is present,
and will be past, where the verbal tenses of the verb
‘to be’ in this claim are not to be analyzed away (just
as the apparent references to the putative A properties pastness,
presentness, and futurity are not to be analyzed away in favor of
reference to B relations).

Thus the standard A Theorist's response to McTaggart's argument
involves the notion that we must “take tense seriously,”
in the sense that there is a fundamental distinction between (for
example) saying that xisF and saying
that xwasF. The thesis can be put this
way.

Taking Tense Seriously: The verbal tenses
of ordinary language (expressions like ‘it is the case
that’, ‘it was the case that’, and ‘it will be
the case that’) must be taken as primitive and
unanalyzable.[3]

In virtue of her commitment to Taking Tense Seriously, the A Theorist
will say that no time ever possesses all of the different A
properties. Thus, according to the A Theorist, there is no
contradiction in the A series — i.e., no contradiction in
saying of a time, t, that t was future, is present,
and will be past — and, hence, no contradiction to be passed
along to the different times at which t was future, is
present, and will be past.

In effect, then, the typical A Theorist makes exactly the move in
response to McTaggart's argument that McTaggart anticipated, and
explicitly rejected. Not surprisingly, then, many supporters of
McTaggart's argument feel that the A Theorist's response fails.

Although some B Theorists deny that time really passes as a result of
considering McTaggart's argument, many B Theorists have different
reasons for saying that time doesn't really pass. Two other arguments
against The A Theory (besides McTaggart's argument, that is) have
been especially influential. The first of these is an argument from
the special theory of relativity in physics. According to that theory
(the argument goes), there is no such thing as absolute simultaneity.
But if there is no such thing as absolute simultaneity, then there
cannot be objective facts of the form “t is
present” or “t is 12 seconds past”. Thus,
according to this line of argument, there cannot be objective facts
about A properties, and so the passage of time cannot be an objective
feature of the world.

It looks as if the A Theorist must choose between two possible
responses to the argument from relativity: (1) deny the theory of
relativity, or (2) deny that the theory of relativity actually
entails that there can be no such thing as absolute simultaneity.
Option (1) has had its proponents (including Arthur Prior), but in
general has not proven to be widely popular. This may be on account
of the enormous respect philosophers typically have for leading
theories in the empirical sciences. Option (2) seems like a promising
approach for A Theorists, but A Theorists who opt for this line are
faced with the task of giving some account of just what the theory of
relativity does entail with respect to absolute
simultaneity. (Perhaps it can be plausibly argued that while
relativity entails that it is physically impossible to
observe whether two events are absolutely simultaneous, the
theory nevertheless has no bearing on whether there is such
a phenomenon as absolute simultaneity.)

The second of the two other influential arguments against The A
Theory concerns the rate of the alleged passage of time. According to
this argument, if it is true to say that time really passes, then it
makes sense to ask how fast time passes. But (the argument goes) if
it makes sense to ask how fast time passes, then it is possible for
there to be a coherent answer to that question. Yet, according to the
argument, there is no rate that can be coherently assigned to the
passage of time. (“One hour per hour,” for example, is
said not to be a coherent answer to the question “How fast does
time pass?”) Thus, the argument concludes, it cannot be true to
say that time really passes.

This argument raises important questions concerning the correct way
to talk about rates, but it has been argued that the A Theorist can
answer those questions in a way that allows her to avoid any untoward
consequences.

According to The B Theory, time is very much like the dimensions of
space. Just as there are no genuine spatial properties (like
being north), but, rather, only two-place, spatial relations
(like north of), so too, according to the B Theorist, there
are no genuine A properties. According to The A Theory, on the other
hand, time is very different from the dimensions of space. For even
though there are no genuine spatial properties like being
north, there are, according to the A Theorist, genuine A
properties; and time, unlike space, can truly be said to pass,
according to The A Theory.

There is another important respect in which some (but not all) A
Theorists believe time to be unlike the dimensions of space. Some A
Theorists believe that there are crucial ontological differences
between time and the dimensions of space. For some A Theorists also
endorse a view known as “Presentism,” and others endorse
a view that we will call “The Growing Universe Theory.”

Presentism is the view that only present objects exist. More
precisely, it is the view that, necessarily, it is always true that
only present objects exist. (At least, that is how the name
‘Presentism’ will be used here. Some writers have used
the name differently. Note that, unless otherwise indicated, what is
meant here by ‘present’ is temporally present,
as opposed to spatially present.) According to Presentism,
if we were to make an accurate list of all the things that exist
— i.e., a list of all the things that our most unrestricted
quantifiers range over — there would be not a single
non-present object on the list. Thus, you and the Taj Mahal
would be on the list, but neither Socrates nor any future Martian
outposts would be included. (Assuming, that is, both (i) that each
person is identical to his or her body, and (ii) that Socrates's body
ceased to be present — thereby going out of existence,
according to Presentism — shortly after he died. Those who
reject the first of these assumptions should simply replace the
examples in this article involving allegedly non-present people with
appropriate examples involving the non-present bodies of those
people.) And it's not just Socrates and future Martian outposts,
either — the same goes for any other putative object that lacks
the property of being present. All such objects are unreal, according
to Presentism.

Presentism is opposed by Non-presentism, which is the view that there
are some non-present objects. More precisely, Non-presentism is the
view that, possibly, it is sometimes true that there are some
non-present objects.

‘Non-presentism’ is an umbrella term that covers several
different, more specific versions of the view. One version of
Non-presentism is Eternalism, which says that objects from both the
past and the future exist just as much as present objects. According
to Eternalism, non-present objects like Socrates and future Martian
outposts exist right now, even though they are not currently present.
We may not be able to see them at the moment, on this view, and they
may not be in the same space-time vicinity that we find ourselves in
right now, but they should nevertheless be on the list of all
existing things.

It might be objected that there is something odd about attributing to
a Non-presentist the claim that Socrates exists right now, since
there is a sense in which that claim is clearly false. In order to
forestall this objection, let us distinguish between two senses of
‘x exists now’. In one sense, which we can call
the temporal location sense, this expression is synonymous
with ‘x is present’. The Non-presentist will
admit that, in the temporal location sense of ‘x
exists now’, it is true that no non-present objects exist right
now. But in the other sense of ‘x exists now’,
which we can call the ontological sense, to say that
x exists now is just to say that x is now in the
domain of our most unrestricted quantifiers, whether x
happens to be present, like you and me, or non-present, like
Socrates. When we attribute to Non-presentists the claim that
non-present objects like Socrates exist right now, we commit the
Non-presentist only to the claim that these non-present objects exist
now in the ontological sense (the one involving the most unrestricted
quantifiers).

According to the Eternalist, temporal location matters not at all
when it comes to ontology. But according to a somewhat less popular
version of Non-presentism, temporal location does matter when it
comes to ontology, because only objects that are either past or
present — but not objects that are future — exist. On
this view, which we can call “The Growing Universe
Theory,” the universe is always increasing in size, as more and
more things are added on to the front end (temporally speaking).

Despite the claim by some Presentists that theirs is the common sense
view, it is pretty clear that there are some major problems facing
Presentism (and, to a lesser extent, The Growing Universe Theory; but
in what follows we will focus on the problems facing Presentism). One
problem has to do with what appears to be perfectly meaningful talk
about non-present objects, such as Socrates and the year 3000. If
there really are no non-present objects, then it is hard to see what
we are referring to when we use expressions such as
‘Socrates’ and ‘the year 3000’.

Another problem for the Presentist has to do with relations involving
non-present objects. It is natural to say, for example, that Abraham
Lincoln was taller than Napoleon Bonaparte, and that World War II was
a cause of the end of The Depression. But how can we make sense of
such talk, if there really are no non-present objects?

A third problem for the Presentist has to do with the very plausible
principle that for every truth, there is a truth-maker. The problem
is that it is hard to see what the truth-makers could be for such
truths as that there were dinosaurs and that there will be Martian
outposts.

Finally, the Presentist, in virtue of being an A Theorist, must deal
with the arguments against The A Theory that were discussed above.

We are all familiar with time travel stories, and there are few among
us who have not imagined traveling back in time to experience some
particular period or meet some notable person from the past. But is
time travel even possible?

One question that is relevant here is whether time travel is
permitted by the prevailing laws of nature. This is presumably a
matter of empirical science (or perhaps the correct philosophical
interpretation of our best theories from the empirical sciences). But
a further question, and one that falls squarely under the heading of
philosophy, is whether time travel is permitted by the laws of logic
and metaphysics. For it has been argued that various absurdities
follow from the supposition that time travel is (logically and
metaphysically) possible. Here is an example of such an argument.

(1)

If you could travel back in time, then you could kill your
grandfather before your father was ever conceived. (For what's to stop
you from bringing a gun with you and simply shooting him?)

(2)

It's not the case that you could kill your grandfather before your
father was ever conceived. (Because if you did, then you would ensure
that you never existed, and that is not something that you
could ensure.)

(3)

You cannot travel back in time.

Another argument that might be raised against the possibility of time
travel depends on the claim that Presentism is true. For if
Presentism is true, then neither past nor future objects exist. And
in that case, it is hard to see how anyone could travel to the past
or the future.

A third argument, against the possibility of time travel to the past, has to do with the claim that backward causation is impossible. For if there can be no backward causation, then it is not possible that, for example, your pushing the button in your time machine in 2020 can cause your appearance, seemingly out of nowhere, in, say, 1900. And yet it seems that any story about time travel to the past would have to include such backward casuation, or else it would not really be a story about time travel.

Despite the existence of these and other arguments against the
possibility of time travel, there may also be problems associated
with the claim that time travel is not possible. For one
thing, many scientists and philosophers believe that the actual laws
of physics are in fact compatible with time travel. And for another
thing, as I mentioned at the beginning of this section, we often
think about time travel stories; but when we do so, those thoughts do not have the characteristic, glitchy feeling that is normally associated with considering an impossible story. To get a sense of the relevant glitchy feeling, consider this story: Once upon a time there was a young girl, and two plus two was equal to five. When one tries to consider that literary gem, one mainly has a feeling that something has gone wrong (one immediately wants to respond, “No, it wasn’t”), and the source of that feeling seems to be the metaphysical impossibility of the story being told. But nothing like this happens when one considers a story about time travel (if it is one of the logically consistent stories about time travel, that is, such as the one depicted in the movie Los Cronocrímenes (Timecrimes)). One task facing the philosopher who claims that
time travel is impossible, then, is to explain the existence of a
large number of well-known stories that appear to be specifically
about time travel, and that do not cause any particular cognitive dissonance.

It is uncontroversial that physical objects are typically extended in
both space and time. But there is some controversy in the philosophy
of time over whether extension in time is analogous to extension in
space. Spatial extension is normally thought of as necessarily
involving different spatial parts at different locations in space.
(Although it should be noted that those who believe in extended
mereological simples (i.e., objects without proper parts)
would deny this.) A bicycle, for example, can be extended across a
doorway in virtue of having some spatial parts inside the doorway and
other spatial parts outside the doorway. Is temporal extension
necessarily like this? That is, when a bicycle is extended from time
t1 to time t2, does it have
its temporal extension in virtue of having different temporal
parts at the different times? Or does a bicycle manage to be
extended in time from t1 to
t2 in virtue of being “wholly
present” (as opposed to merely partly present) at each of those
times?

According to The 4D View, temporally extended objects have temporal
parts, temporal extension is perfectly analogous to spatial
extension, and time is one of four dimensions that are on a par, at
least with respect to the manner in which objects are spread out in
space-time. On The 3D View, however, temporally extended objects do
not have temporal parts, temporal extension is very different from
spatial extension, and time is unique among the four dimensions of
the world, at least with respect to the manner in which objects are
spread out in space-time.

On The 4D View, objects are to be thought of as four-dimensional
“space-time worms,” each of which is made up of many
different temporal parts, like the different spatial segments of an
earthworm. An object at a time — Descartes in 1625, for example
— is not the whole object but, rather, a mere part (a temporal
part) of that object; and the relation between Descartes in 1625 and
Descartes in 1635 is like the relation between the two wheels of a
bicycle: they are different parts of a bigger whole. By contrast, on
The 3D View, objects are to be thought of as three-dimensional things
that are not made up of different temporal parts. On this view, an
object at a time — Descartes in 1625, for example — is
the same thing as the whole object — Descartes. Thus, according
to The 3D View, the relation between Descartes in 1625 and Descartes
in 1635 is the relation of identity: each one is just the same thing
as Descartes.

As in the case of the disputes between A Theorists and B Theorists,
on the one hand, and Presentists and Non-presentists, on the other
hand, the 3D/4D controversy is part of a general disagreement among
philosophers of time concerning the degree to which time is
dissimilar from the dimensions of space. That general disagreement
has been an important theme in the philosophy of time during the last
one hundred years, and will most likely continue to be so for some
time to come.

Earman, John, 1995, “Recent Work on Time Travel,” in
Steven Savitt (ed.), Time's Arrows Today: Recent Physical and
Philosophical Work on the Direction of Time, Cambridge: Cambridge
University Press, pp. 268–310.

–––, forthcoming, “The Truth About the Past and
the Future,” in Fabrice Correia, and Andrea Iacona
(eds.), Around the Tree: Semantic and Metaphysical Issues
Concerning Branching Time and the Open Future, Dordrecht:
Springer.

Savitt, Steven, 2000, “There's No Time Like the Present (in
Minkowski Spacetime),” Philosophy of Science
(Supplementary Volume: Proceedings of the 1998 Biennial Meetings of
the Philosophy of Science Association), 67: 5563–5574.

Savitt, Steven (ed.), 1995, Time's Arrows Today: Recent
Physical and Philosophical Work on the Direction of Time,
Cambridge: Cambridge University Press.

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